Attack on U.S. mission in Benghazi – Attackers set the U.S. mission in Benghazi, Libya, on fire on September 11, 2012. The U.S. ambassador to Libya, Christopher Stevens, and three other U.S. nationals were killed during the attack. The Obama administration initially thought the attack was carried out by an angry mob responding to a video, made in the United States, that mocked Islam and the Prophet Mohammed. But the storming of the mission was later determined to have been a terrorist attack.

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Photos: Attack on U.S. mission in Benghazi22 photos

Attack on U.S. mission in Benghazi – Obama and Clinton stand at Andrews Air Force Base as the bodies of the four Americans killed are returned on September 14.

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Attack on U.S. mission in Benghazi – A desk sits inside the burnt U.S. mission on September 13, two days after the attack.

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Attack on U.S. mission in Benghazi – Damage is seen inside the U.S. mission on September 13.

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Attack on U.S. mission in Benghazi – A lounge chair and umbrella float in the swimming pool of the U.S. mission on September 13.

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Attack on U.S. mission in Benghazi – Demonstrators gather in Libya on September 12 to condemn the killers and voice support for the victims.

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Attack on U.S. mission in Benghazi – U.S. President Barack Obama, with Secretary of State Hillary Clinton on September 12, makes a statement at the White House about Stevens' death.

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Attack on U.S. mission in Benghazi – A burnt vehicle is seen at the U.S. mission in Benghazi on September 12.

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Attack on U.S. mission in Benghazi – People inspect the damage on September 12.

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Attack on U.S. mission in Benghazi – A small American flag is seen in the rubble on September 12.

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Attack on U.S. mission in Benghazi – A man stands in part of a burned-out building of the U.S. mission on September 12.

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Attack on U.S. mission in Benghazi – Smoke and fire damage is evident inside a building on September 12.

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Attack on U.S. mission in Benghazi – Half-burnt debris and ash cover the floor of one of the U.S. mission buildings on September 12.

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Attack on U.S. mission in Benghazi – The U.S. mission is seen in flames on September 11, the day of the attack.

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Attack on U.S. mission in Benghazi – A protester reacts as the U.S. mission burns on September 11.

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Attack on U.S. mission in Benghazi – A vehicle and the surrounding area are engulfed in flames on September 11.

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Attack on U.S. mission in Benghazi – Flames erupt outside of a building on September 11.

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Attack on U.S. mission in Benghazi – A vehicle burns during the attack on the U.S. mission on September 11.

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Attack on U.S. mission in Benghazi – Onlookers record the damage from the attack on September 11.

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Attack on U.S. mission in Benghazi – Onlookers walk past a burning truck and building on September 11.

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Attack on U.S. mission in Benghazi – A vehicle sits smoldering in flames on September 11.

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Attack on U.S. mission in Benghazi – People duck flames outside a building on September 11.

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Story highlights

CIA has said contractors not required to sign nondisclosure forms about Benghazi

But questions are raised about the timing, intent of the forms

Contractors who responded to terror attacks in Libya tell their story to Congress

Ambassador Christopher Stevens and three other Americans were killed

CIA contract security officers who responded to the terrorist attacks on the U.S. ambassador in Benghazi, Libya, last year were told by the CIA to sign nondisclosure agreements just before a memorial ceremony at the CIA's headquarters in Langley, Virginia, this past May, a congressional source told CNN.

The source says this was disclosed by the contractors during closed-door testimony on Capitol Hill this week, and it seems to contradict information sent by CIA Director John Brennan to the House Intelligence Committee investigating the events in Benghazi.

The CIA has repeatedly denied reports by CNN that the CIA has tried to prevent its personnel from talking to members of Congress investigating the September 2012 Benghazi attacks.

When it was first reported by CNN, Brennan released a letter showing that he strongly encouraged CIA operatives to cooperate with congressional investigators. Brennan even went so far as to directly answer a question from House Intelligence Committee Chairman Mike Rogers who, in a September 3 letter to the CIA director, specifically asked this question:

"Has any officer, either staff of contractor, been required to sign any nondisclosure agreement because of their presence at Benghazi or their participation in any activity related to the Benghazi attacks?"

"CIA contractors routinely sign secrecy agreements, which are standard forms. No CIA officer has ever signed a secrecy agreement that referenced Benghazi or that prohibited them from talking to Congress," Ebitz said in a statement.

"In fact, CIA secrecy agreements specifically note an officer's right to bring issues to the attention of Congress. Furthermore, Director Brennan extended to all Benghazi survivors an invitation to speak to Congress and indicated the Agency would support their interaction. Several have spoken to CIA's oversight committees."

All five operatives were part of a security team hired by the CIA to protect its operations during missions in Libya.

The official nondisclosure forms did not specifically mention the Benghazi investigation, but the source told CNN, "There is not a person in Washington, D.C. who doesn't understand why the forms were put in front of these people."

The Weekly Standard first reported the signing of additional nondisclosure agreements on Thursday.

Mark Zaid, an attorney representing three of the CIA contractors, told the Weekly Standard, "There is no doubt that the NDAs (nondisclosure agreements) would not have been presented to them had it not been for Benghazi. That is their impression and my analysis based on 20 years of experience."

Zaid continued, "the NDAs in no way changed the legal landscape. They had already signed NDAs that bound them to certain obligations. These new ones were legally unnecessary."

The CIA contractors told to sign the nondisclosure agreements were attending a memorial service for Tyrone Woods and Glen Doherty, two CIA operatives who died in the Benghazi attacks.

This week, the House Intelligence Subcommittee on Oversight met to hear closed-door testimony from the former security agents.

Though the testimony is secret, the congressional source with inside knowledge of the hearing told CNN, "There now appears to be serious discrepancies" between what the CIA has officially reported to Congress and what the witnesses are telling members of the committee.

CNN has learned a rescue team of five CIA operatives were armed and ready to respond to the terrorist attack within minutes of the first radio communication from Ambassador Christopher Stevens' compound, but CNN sources say the agents were told to stand down.

The allegation was first raised last year, after which the CIA told Congress there was no order to stand down. The CIA has said "no one at any level in the CIA told anybody not to help those in need; claims to the contrary are simply inaccurate."

Members of the House Intelligence Subcommittee believe they need to have more information to determine why a discrepancy now exists between the would-be rescuers on the ground and the official reports from the CIA.

The agents eventually did respond to the attack, pulling off a heroic attempt to save U.S. State Department officials who were under siege.

Intelligence officials explained last year that there was a roughly 25-minute gap from the initial call for help to when half a dozen officers were able to make a move -- and that the delay was due to the time it took to load weapons and equipment and coordinate with friendly militias.